1. Field of the Invention
This invention relates to conductive polymer compositions and electrical devices comprising them.
2. Background of the Invention
Conductive polymers and electrical devices such as self-regulating heaters comprising them are well-known.
Reference may be made, for example, to U.S. Pat. Nos. 3,861,029, 4,177,376, 4,188,276, 4,237,441, 4,304,987, 4,388,607, 4,426,339, 4,514,620, 4,534,889, 4,545,926, 4,689,475, and 4,719,335, European Patent Publication No. 38,718 (Fouts, et al), and copending, commonly assigned application Ser. Nos. 711,909 filed Mar.14, 1985,(Deep, et al) now U.S. Pat. No. 4,761,541, 818,846 filed Jan. 14, 1985 (Barma) now abandoned, 75,929 filed July 21, 1987 (Barma, et al) and 202,165 Oswal, et al.) filed contemporaneously with this application, the disclosures of which are incorporated herein by reference. As a result of a PTC (positive temperature coefficient of resistance) anomaly, such compositions can be used in electrical devices to provide temperature control over a narrow temperature range, resulting in "automatic" shutdown in the event of exposure to overtemperature or overvoltage conditions or "automatic" heating when exposed to a colder environment.
Conductive polymer compositions can made in a wide range of resistivities in order to meet the requirements for a specific application. For example, compositions for circuit protection devices, which are normally powered at voltages of 10 to 600 volts, may have resistivities of 0.001 to 100 ohm-cm. Strip heaters designed to be powered at 120 to 240 volts have routinely been made from compositions with resistivities of 1,000 to 50,000 ohm-cm. Laminar resistance heaters which may have a small distance between the electrodes and thus a short current path may require compositions with resistivities of 500 to 500,000 ohm-cm. Using traditional conductive fillers such as carbon black, it is difficult to make high resistivity conductive polymer compositions, i.e. those with a resistivity of more than 10,000 ohm-cm, reproducibly. FIG. 1 shows a loading curve for a conductive polymer: the resistivity on a log scale is plotted as a function of the percent by volume of filler. For a filler of a given resistivity, the polymer is relatively nonconductive until a threshold filler loading is reached (region A). In region B, the resistivity decreases rapidly as the filler concentration increases. The sensitivity of the resistivity to filler loading is relatively low in region C. For conductive polymer compositions which have high resistivities and a low concentration of filler, small errors during the weighing of the ingredients or inconsistencies during mixing will have a significant effect on the resistivity of the final composition.
A second issue for conductive polymer compositions is that of thermal stability. During the normal operation of devices comprising conductive polymers it is common for the polymer to be exposed to a variety of thermal conditions, either as a result of the device self-heating or due to changes in the ambient temperature. In the case of heaters, it is common for the PTC element comprising the conductive polymer to undergo a large number of thermal cycles from low temperature to elevated temperatures. These elevated temperatures may be equal to or greater than the melting point, Tm, of the polymer matrix in the conductive polymer. (Tm is defined as the temperature at the peak of the melting curve of the conductive polymer as measured by a differential scanning calorimeter.) Although it is common for the polymer to undergo changes in resistivity as a result of oxidation or relaxation when exposed to elevated temperatures, for cost applications these resistivity changes are not desirable. For instance, heaters are expected to produce a specific power output at a given voltage. As the resistance increases, the power will decrease. It is particularly undesirable for the resistance to change each time the heater is exposed to an elevated temperature. Alternatively, circuit protection devices must be stable so that the switching current is not adversely affected.
A number of proposals for producing high resistivity compositions and/or increasing the thermal and electrical stability of conductive polymer compositions have been made. In several cases, conductive fillers which have a higher resistivity than conventional conductive fillers have been used. If a greater quantity (i.e. higher loading) of filler is required to generate a comparable resistivity, the sensitivity of the loading curve can be minimized.
U.S. application Ser. Nos. 818,846 filed Jan. 14, 1985 (Barma) and 75,929 filed July 21, 1987 (Barma now abandoned, et al.) disclose conductive polymer compositions in which the particulate conductive filler distributed in the polymer matrix itself comprises a conductive polymer in which a second particulate filler is distributed in a polymer matrix.
Japanese Patent Application No. 49-134096 (published as No. 51-59947) discloses conductive compositions comprising a crystalline organic polymer having dispersed therein conductive particles which have a resistivity of less than 1 ohm-cm (e.g. carbon black or silver) and 1 to 20% by volume of inorganic particles (e.g. zinc oxide, cadmium sulfide, or silicon, or other meal oxides). These compositions are suitable for use in photometers, thermistors, and magnetometers. Japanese Patent Application No. 54-78745 discloses a PTC composition which comprises a polymer matrix having dispersed therein conductive particles (e.g. graphite or carbon black) and semiconductive particles (e.g. a metal oxide or organic semiconductor such as TCNQ) in a volume ratio of 0.25:4.0. None of these publications defines the specific particle sizes and ratios of the fillers necessary to provide thermal stability in a PTC conductive polymer composition.
European Patent Publication No. 38,718 discloses the use of non-conductive particulate fillers, i.e. those with a resistivity greater than 1.times.10.sup.6, to improve the thermal stability of conductive compositions comprising carbon black. In preferred formulations the volume loading of the non-conductive filler is less than that of the carbon black.
U.S. Pat. No. 4,545,926 discloses conductive polymer compositions in which the electrical stability, as measured by current transients, is improved by the addition of a nonmetallic filler to a polymer/metal blend.